Differential equation with sphere

A spherical mothball of original radius 1.2 cm slowly
evaporates such that 180 days later, its radius is only 1 cm.
Physically, the rate of evaporation dr/dt is proportional to the
surface area of the sphere. Determine a) the time required for
the radius of a new mothball to shrink to 25 percent of its
original radius, and b) the time required for the volume of a
new mothball to become half of its original value.

2. Relevant equations

SA= 4 pi r^2
r(0)=1.2
r(180)=1

3. The attempt at a solution

I started by saying -dr/dt = c1 4 pi r^2

where c1 is a constant of proportionality

Then through separation of variables I found that

1/r = 4 pi c1 t + c2

after imposing the initial conditions I found c1=7.383*10^-5 and c2=.833

A spherical mothball of original radius 1.2 cm slowly
evaporates such that 180 days later, its radius is only 1 cm.
Physically, the rate of evaporation dr/dt is proportional to the
surface area of the sphere. Determine a) the time required for
the radius of a new mothball to shrink to 25 percent of its
original radius, and b) the time required for the volume of a
new mothball to become half of its original value.

2. Relevant equations

SA= 4 pi r^2
r(0)=1.2
r(180)=1

3. The attempt at a solution

I started by saying -dr/dt = c1 4 pi r^2

where c1 is a constant of proportionality

Then through separation of variables I found that

1/r = 4 pi c1 t + c2

after imposing the initial conditions I found c1=7.383*10^-5 and c2=.833